It is known to convert ethylene based copolymers made using a metallocene catalyst, such as in gas phase or solution or high pressure polymerization, into film by blown film extrusion and cast film extrusion in either a mono or multilayer structure. Such copolymers are sold under the trade name EXACT or EXCEED by ExxonMobil Chemical Company. The composition for the film may comprise a single polymer with the usual additives (anti-oxidant, anti-block additive etc.) It is also known to blend different polymers. An example of that is the use of varying amounts of low density polyethylene made in a high pressure free radical initiated process, referred to as LDPE, with ethylene based copolymers, whether made using metallocene catalyst or the conventional catalyst types produced using titanium chloride as the transition metal component. In the LDPE'S have broad molecular weight distributions and contain significant long chain branching (LCB). In the density range of from 0.91 g/cm3 to 0.94 g/cm3 such copolymers are often referred to as linear low-density polymers, LLDPE's for short.
The conversion of the copolymer composition into film can proceed by two principal routes: A) blown film extrusion, which requires melt strength to sustain the bubble formed as it cools and the polymer composition solidifies; and B) cast film where the molten polymer is cooled on a chilled metal roll. Melt strength is influenced by the molecular weight (the lower the Melt Index (MI) the higher the melt strength at the same extrusion temperature) and LCB. Low MI's of around 1 g/10 min have been favored to achieve bubble stability in blown film extrusion. MI's of over 2.5 g/10 min are favored in cast extrusion to achieve good flowability and reduced neck in. The films produced are frequently converted on the packaging line into some sort of containment structure referred to herein as a bag, which may be a pouch, a bread bag or any other type of bag. A form, fill and seal machine may be used to form the bag.
Once the film has been formed, a critical performance factor is the speed at which the packaging line can be operated. On most bag producing machines, the bottleneck that limits further speed increases of the packaging line is the speed at which the film can be sealed to form the bag. The most important indicators for high line-speed potential are:
A) the heat seal strength (HSS) at different temperatures;
B) the hot tack, which influences the time taken before the product to be packed can be dropped on the freshly made seal and ensure package integrity; and
C) the seal initiation temperature (SIT), which determines the lowest temperature at which sufficient heat seal strength is developed to keep the bag closed.
A higher heat seal strength and/or a broader hot tack would provide a broader operating window, lowering the SIT and decreasing the heat seal cycle time and so increase the line speed with which the machine can reliably bag the products. It is among the objects of the invention to broaden the operating window while maintaining a reasonable balance of other properties for processability and film formation.